Thin-film devices include, for example, thin-film solar cells, liquid crystal panels, semiconductor devices, and so on. For example, in the field of thin-film solar cells, except for photoelectric-conversion electricity generating layers (semiconductor material such as silicon) that generate electrical current upon absorption of light, transparent conductive films and transparent optical films that form collector electrodes at the light incident side are used.
Here, taking the example of a tandem solar cell, the balance of the generated currents in a top cell and a bottom cell is corrected by forming a thin-film called an intermediate contact layer between the top cell and the bottom cell. For example, the top cell is a photoelectric conversion layer formed of amorphous silicon, the intermediate contact layer is a transparent conductive film, and the bottom cell is a photoelectric conversion layer formed of crystalline silicon.
The intermediate contact layer mentioned above reflects part of the sunlight passing transmitted through the top cell and returns it to the top cell, and also transmits the remaining light and guides it to the bottom cell. Hence, by providing the intermediate contact layer, part of the light can be returned to the top cell, and therefore, the film thickness of the top cell can be reduced while still maintaining the photoelectric effect. The ability to reduce the film thickness of the top cell is preferable from the standpoint of photodegradation, and by providing the intermediate contact layer, it is possible to improve the performance of the entire solar cell.
In such a tandem solar cell, it is necessary to make the generated currents in the top cell and the bottom cell uniform. These generated currents are adjusted by the material (refractive index) and the film thickness of the intermediate contact layer. Because the film thickness of the intermediate contact layer is thin, about 10 nm to 150 nm, if it falls outside of a suitable range, the balance of the generated currents in the top cell and the bottom cell is lost, and therefore, the generated current from the tandem solar cell is reduced, and the cell performance decreases. Therefore, in the related art, film-thickness control of this intermediate contact layer is essential, and random inspection is performed from the production line.
The transparent conductive film and the transparent optical film are not limited to the intermediate contact layer in the tandem solar cell described above; they can be used in various fields such as thin-film solar cells and multijunction thin-film solar cells, as well as electrodes of semiconductor devices such as thin-film transistors, liquid-crystal driving electrodes of liquid crystal panels, and so forth, which are subjected to film-thickness inspection as required.
A known apparatus for measuring film thickness in the related art is, for example, an interferometric film-thickness measurement apparatus in which a thin film to be measured is irradiated with light, interference of reflected light thereof and light reflected at a rear surface of the thin film is split into each wavelength, a spectral intensity distribution of the split wavelengths is created, and the film thickness is measured on the basis of this distribution (for example, see Patent Document 1).
Patent Document 1:    Japanese Unexamined Patent Application, Publication No. HEI-10-311708